Respiratory Histology The function of the respiratory system is to provide molecular oxygen for cellular oxidation and to remove carbon dioxide generated as a waste product of cell metabolism. There must be vascular or circulatory transport AND air/gas transport

The pulmonary blood circulation transports gases to and from the alveoli of the lungs.

Functional Requirements The Respiratory System has tubular components for air/gas conduction Plus respiratory components for gas exchange. Inspiratory pressure tends to collapse the conducting tubes. They must be held open. Other essential functions include: warming; moistening; cleansing by removal of particulate matter; detoxification by absorption of harmful gases; and entrapment of harmful bacteria and viruses.

Conduction The nasal cavity is held open by bone and cartilage. Most of the additional functions are carried out by pseudostratified columnar ciliated epithelial cells and goblet cells (mucous secretors) that line the airways. The cilia beat rhythmically in one direction only, moving debris and pathogen-laden mucous to the oropharynx and mouth where it is expectorated or swallowed. Air temperature control is by the profuse capillary beds that are beneath the epithelium, and which warm or cool inspired air. Extensive venous plexi largely replace capillaries in the nasal cavities to modify air temperature.

Trachea The trachea typifies the conducting system. The cilia are paralysed by cigarette smoke. At rest, smooth muscle contraction decreases tracheal (and bronchial) diameter to decrease respiratory dead space. Submucosal glands Smooth muscle to control diameter (trachealis) Fibro-elastic support Pseudostratified ciliated columnar epithelium Goblet cell (mucous) Cartilage C- rings hold trachea open

Bronchus and Bronchial Tree The bronchus is similar to the trachea, but branching into the lungs, and the C-shaped cartilage rings are replaced by plates. The epithelium is still pseudostratified, ciliated columnar, but height decreases. Epithelium Cartilage plate Cartilage plate Glands Adventitia

The bronchial tree divides and eventually forms bronchioles with a lumen of 1mm or less in diameter. The epithelium becomes ciliated columnar. Cartilage and glands disappear and the bronchiole is held open by the surrounding lung tissue. The smooth muscle in the wall may excessively narrow the lumen in asthma. Smooth muscle Bronchiole Lung tissue

Bronchioles continue to divide and decrease in size, becoming terminal, and then respiratory, which give the alveolar ducts, alveolar sacs and alveoli.

Terminal and Respiratory Bronchioles Epithelium becomes non-ciliated cuboidal and goblet cells disappear. Gas exchange begins to occur in the respiratory alveoli that bud from the respiratory bronchioles. Alveolar duct Respiratory bronchiole Terminal bronchiole Lung alveoli

Alveoli The alveoli are the functional unit of the lung where gaseous exchange takes place. They are out-pocketings of respiratory bronchioles, alveolar ducts and alveolar sacs. The most conspicuous feature of the alveolar wall is the presence of many small capillaries in the septae between them. Opposite the septae the alveoli are open to allow air entry.

Epithelium Two types of epithelial cells line (form) the wall of the alveoli. Type I are the flattened epithelial cells (squamous pneumocytes) that have cytoplasmic extensions about 0.1 - 0.3 um thick and have no microvilli. In some areas the basal lamina of the type I cell is fused to the capillary endothelium - possible areas of gas exchange. Type II, the large alveolar cells (granular pneumocytes) are cuboidal without broad cytoplasmic extensions and occupy about 5% of the alveolar surface. Capillary Type I Type II (surfactant) Macrophage or dust cell

Epithelium Type II Cells produce Surfactant, a stabilizing, surface-active material consisting mainly of the phospholipid lecithin. Surfactant is crucial to lung function Its high viscosity and low surface tension stabilises the diameter of alveoli preventing collapse after expiration, while retaining residual air in the alveoli after each breath; because alveoli are already partially open, energy expenditure in subsequent inspirations is much less. Acting as a detergent surfactant facilitates the removal of amniotic fluid from the lungs at birth, before the first breath. Dust cells (macrophages) usually lie free in the alveoli and air passageways. Their role is to clear the lungs of invading bacteria and inspired particulate material (eg dust and carbon particles).

In Summary Hugely branching respiratory “tree” resulting in massive surface area for gas exchange (75 square metres) As lumen decreases so do cartilage, cilia, epithelial height and mucous Smooth muscle controls diameter, but xs narrowing is asthma Surfactant is thick in polycystic fibrosis